Reply #9: The rationale arises from the use of calculators

[jberryhill]

The problem introduced by calculators is that kids bang in numbers, make a keystroke error, and are blissfully unaware that their answer is off by orders of magnitude.

The point of introducing guesstimating was to have them develop a sense of when their use of the calculator has produced a tremendously wrong result. It didn't need to be a "taught skill" in the 70's to that extent.

However, at least by the time they are doing algebra, it's simpler just to have them plug their results back into the problem to check consistency, which is much more important in the long run than a gut sense of basic calculation error, IMHO, but I'm not a math educator.